Russian forces have sunk several coast guard vessels in Georgia's military port of Poti, Al Jazeera's Hoda Abdel Hamid has reported from the scene.
The attack on Wednesday follows a day of dramatic developments in the Russia-Georgia conflict amid what appears to have been an escalation of military action on the ground.
Abdel Hamid said: "Russia is clearly on the offensive.
"We have seen more and more Russian troops coming into the area all day - a continuous build up of forces including columns of tanks and truck all along the roads here.
"They came into this area and destroyed six Georgian vessels.
"From what we understand, they came with the specific task of destroying all the military facilities of the Georgians," she said.
Al Jazeera's Jonah Hull said: "Poti is one of the most important ports in the Black Sea.
"The offensive means that the ceasefire is dead - back to ground zero."
Russian military vehicles earlier headed towards the Georgian capital of Tbilisi, Al Jazeera's Alan Fisher reported.
Fisher tailed a convoy of Russian armoured vehicles on the road from the town of Gori, less than 60km from Tbilisi.
"No more than 100 metres away is an armoured personnel carrier with three Russian soldiers on top ... behind that there is a long convoy of Russian military vehicles," he reported.
"The fact Russian forces have moved so far into Georgian territory will be seen as an act of provocation," Fisher said.
"They're not moving at a fast speed, but they are moving towards Tbilisi.
"It is only 60km from Gori to Tbilisi, but this doesn't look like an invasion force."
He later said the convoy had come to a halt before heading back to Gori.
"Act of provocation"
Georgian and Russian officials have traded accusations that troops from each side have been acting against the spirit of an EU-mediated peace plan agreed in Moscow the previous day.
Al Jazeera's Jonah Hull, reporting from Tbilisi said: "This is an extraordinary show of Russian force.
"This seems to have overstepped the bounds of the Russian side of the ceasefire - they [had] pledged not to move from their positions and to stop firing."
The events came a day after Nicholas Sarkozy, the French president, hailed the EU-mediated ceasefire agreement reached in Moscow between Georgia and Russia.
Aljazeera's Neave Barker, reporting from Moscow, said: "These are disturbing developments given all the work that was done by the EU yesterday."
He said the Russians had agreed with the Georgians on Tuesday to withdraw to the positions they were in before the outbreak of the conflict.
But he added: "We have heard from our colleagues on the ground that this isn't the case."
Earlier the secretary of Georgia's security council told local television that 50 Russian tanks and armoured personnel carriers were in Gori, about 30km from the breakaway region of South Ossetia.
"Russian occupying forces were continuing movements across Georgia despite the ceasefire," Mikheil Saakashvili, Georgia's president, said early on Wednesday as he stood alongside the leaders of other former Soviet states.
| Saakashvili, centre, said Russian forces were "continuing movements" [AFP]
But General Anatoly Nogovitsyn, Russia's deputy chief of staff of the armed forces, said it was Georgian forces that were violating the ceasefire.
"Georgian forces have begun their pullback towards Tbilisi but no active withdrawal has yet been observed," he said.
"We are still obliged to fulfil our mission by taking out firing positions, snipers and so on."
Earlier Al Jazeera's Fisher reported that Russian forces in tanks and armoured personnel carriers had essentially "taken over the town of Gori".
"I myself saw three personnel carriers on the edge of the town ... but there was no doubt they were moving towards Gori," he said.
However, Russia's military on Wednesday repeatedly denied that any troops were inside Gori.
"Neither Russian peacekeepers nor any units subordinate to them are present in Gori," the Interfaxagency quoted a Russian military spokesman as saying.
Sergei Lavrov, Russia's foreign minister, said that a commitment in the peace plan drafted by the French under which Georgian forces would withdraw to "permanent positions" meant they should return to barracks.
|Russia's foreign ministers says troops will stay until Georgia withdraws [AFP]
"Upon the withdrawal of Georgian troops to their barracks, Russian troops will return to the territory of the Russian Federation," Lavrov said.
"Our peacekeepers will remain in South Ossetia," he said, referring to peacekeeping units separate from the regular army that have been deployed in the Georgian province for 15 years.
Saakashvili said on Wednesday that Russia was responsible for "Balkan-type and World War II-type ethnic cleansing and purification campaigns" during the conflict.
"We are getting reports of large-scale violation of human rights of the worst of the case," he said.
"The town of Tskhinvali was turned into Grozny Two by Russian carpet bombardment and I really want people to go in and check and verify what kind of bombs are these.
"I have been hearing accusation that this was Georgian bombing and this is not true."